This invention relates generally to wood pulp bleaching processes and more particularly to wood pulp bleaching processes employing gaseous bleaching reagents.
Wood pulp bleaching with gaseous reagents, such as oxygen and ozone, promise significant reduction of objectionable pulp mill effluent to streams and other bodies of water. Elimination of chlorine compounds from the bleaching sequence promises great economic and ecological benefits. However, incorporation of these bleaching reagents can impose significant capital costs on the pulp mill.
For example, the incorporation of ozone has been hindered in commercial applications to a large extent by high capital costs which are inherent to common prior art bleaching sequences which usually require that an ozone bleaching stage is followed by a pulp washing step. From the washer, the pulp is pumped at medium consistency to a mixer wherein alkaline chemicals, such as caustic soda together with any one of a number of reinforcing agents, eg. oxygen, hydrogen peroxide, sodium hypochlorite, or the like are added to the pulp. At the same time, the pulp is heated to increase its temperature above that at which it was discharged from the ozone reactor. The heated and alkalized pulp is then discharged from the mixer to the alkaline extraction stage.
Addition of the cost of the washer and pump to the cost of an existing bleaching operation in order to incorporate an ozone bleaching stage, when considered together with other difficulties and costs associated with ozone bleaching, often makes ozone bleaching economically undesirable. Any reduction of capital equipment requirements clearly would improve the acceptance of ozone bleaching and would increase its use.
The foregoing illustrates limitations known to exist in present devices and methods. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.
In one aspect of the present invention, this is accomplished by a method for transporting pulp to subsequent bleaching stages from an initial gaseous bleaching stage for medium consistency pulp, including retaining gas pressure of the initial bleaching stage, discharging pulp from the initial bleaching stage under retained gas pressure, and allowing the retained gas pressure to transport the pulp to a mixer and onward through subsequent stages.
The foregoing and other aspects will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawing figures.